U.S. Pat. No. 5,763,483, which is herein incorporated by reference, disclosed carbocyclic compounds and pharmaceutically acceptable salts thereof. Among them Oseltamivir, chemically Ethyl (3R,4R,5S)-4-(acetylamino)-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate is a orally active inhibitor of influenza virus neuraminidase. Oseltamivir is represented by the following structure:

Various processes for preparation of oseltamivir were disclosed, for example, in U.S. Pat. No. 5,763,483, J. Org. Chem., Vol. 63, No. 13, 1998 (page: 4545-4550), J. Amer. Chem. Soc., Vol. 115, No. 4, 1997 (Page: 681-690), U.S. Pat. No. 5,952,375 and PCT Publication No. WO 99/44185.
In the preparation of oseltamivir, the compound of formula B:
is a key intermediate. According to the prior art processes, the intermediate of the formula B was prepared by acetylation of the compound of formula A:
with acetic anhydride in hexanes or methylene chloride in the presence of aqueous sodium bicarbonate; or with acetyl chloride and pyridine. The base such as sodium bicarbonate is normally used to convert the acetic acid formed as by-product into a water soluble salt such as sodium acetate and the salt formed is extracted into water present so as to move the equilibrium towards the formation of acetylated product of the formula B.
We have surprisingly found, in contrary to the prior art process and process normally followed, that acetylation reaction of compound of formula A proceeds with acetic anhydride in an organic solvent in the presence of an organic or inorganic base in the absence of water, cleanly to obtain acetyl derivative of the formula B in better purity and in better yield.
In the preparation of oseltamivir, the azide intermediate of formula B is reduced to the corresponding amine, (oseltamivir) of the formula C:

According to the prior art process, the reduction was carried out with hydrogen gas and a catalyst such as platinum on carbon or Lindlar's catalyst or reducing reagent such as trialkyl or triaryl phosphine.
We have found that the reduction of the compound of formula B to obtain the compound of formula C may be carried out using hydrogen sulfide or Na2S under suitable conditions in an advantageous manner.